Microbes assemble in diverse communities responsible for crucial functions, such as regulating the response of the biosphere to environmental variation and driving the bulk of elemental cycles. Understanding the factors shaping microbial community assembly is pivotal to predict the functioning of ecosystems in a rapidly changing world and design interventions to mitigate the loss of biodiversity and ecosystem services. In this talk, I will outline mechanistic links between changes in the diversity and structure of microbial communities and variations in fundamental environmental variables, specifically temperature and available nutrients. My work shows the power of combining theory-informed controlled experiments with data from communities in natural habitats to get a mechanistic understanding of the principles governing microbial community assembly.